The purpose of this study is to elucidate the mechanism of destruction of tumor cells by lymphocytes, with particular attention to the role of lymphotoxin (a cytotoxic lymphocyte mediator). Since we have extensively purified human lymphotoxin and the biochemical changes in target cells induced by this mediator. Attempts are being made to modify (increase and decrease) the activity of lymphotoxin, which has definite implications in the area of tumor immunity.